In the packaging of certain types of foods, such as snack foods like potato chips, cookies and the like, it is common practice to employ a multi-layer film. A desirable property in such a packaging film is its opacity which serves to protect the packaging and the product contained therein from deterioration caused by exposure to light. In particular, it has been found that when the package is exposed to certain wavelengths of light, increased spoilage in some packaged products results. Thus, the degree of opacity which is present in the film has been found to be a significant parameter, because it has been found that spoilage of the contents of the package occurs even if the film allows the passage of only some light.
It is known in the art that thermoplastic polymers can be loaded with inert fillers, cast into films, and thereafter stretched to form opaque, oriented thermoplastic films. While this statement is generally true, it should be realized that fulfilling this depends on the particular components employed in formulating the filled film, and on the specific process parameters employed.
In U.S. Pat. Nos. 3,697,367 and 3,795,720, there are disclosed processes for preparing uniaxially oriented mixed polymer systems. The resulting material has utility for being formed into fibers, which can in turn be used for making sheet form material, like paper.
Oriented opaque film compositions are generally known in the art. U.S. Pat. No. 4,377,616 discloses an opaque biaxially oriented polymer film structure comprising a thermoplastic polymer matrix core possessing numerous voids, a substantial number of which contain at least one spherical void-initiating particle. This polymer film structure also comprises transparent thermoplastic skin layers adhering to the surfaces of the core layer. The unique structure of the core layer disclosed in this patent is said to impart a much higher degree of opacity than that which has been found to be possible from the use of an opacifying pigment alone. This is possibly due to the effects of light scattering.
This patent discloses the preparation of a film by melting a mixture of a major proportion of a film forming, first polymer, such as a polypropylene, and a minor proportion of a second polymer, which is incompatible with the first polymer and which has a higher melting point than the first polymer. The mixture is brought to a temperature sufficiently high to melt the incompatible polymer and to disperse it in the film forming first polymer. The melted dispersion mixture is then extruded into a film and biaxially oriented in the conventional manner. Upon solidification in film form, the dispersed incompatible polymer particles provide sites for the formation of voids surrounding these polymer particles which are created during orientation. These voids provide opacity and give the film an attractive pearlescent sheen.
U.S. Pat. No. 4,533,509 discloses a composite oriented structure having at least one thin surface layer containing highly subdivided inorganic material in a proportion which is suited to providing antiblock properties and to reducing the coefficient of friction of the product. The thin surface layer is formed simultaneously or sequentially on a comparatively thick base polymer layer and the composite is thereafter oriented.
U.S. Pat. No. 4,632,869 discloses an opaque, biaxially oriented film structure comprising a polymer matrix with a stratum of voids. The voids contain spherical, void-initiating particles of polybutylene terephthalate. The structure may also include thermoplastic skin layers and may include from about 1% to 3% by weight of a pigment, such as white TiO.sub.2, or colored oxides.
U.S. Pat. No. 4,652,489 discloses an oriented, sealable, opaque polyolefin multilayer film comprising: a core layer containing vacuoles, a sealable surface layer, and a non-sealable surface layer which incorporates a slip agent such as a polydiorganosiloxane.
U.S. Pat. No. 4,741,950 discloses a differential opaque polymer film with a core layer containing numerous microscopic voids, a first wettable skin layer, which has a rough appearance, and which contains an antiblocking agent, such as: silica, a silicate, clay, diatomaceous earth, talc and/or glass; and a second wettable skin layer, which has a smooth appearance and which may be metallized. TiO.sub.2 may be present in the core and/or the first skin layer. The film allows a light transmission of 24%.
U.S. Pat. No. 4,758,462 also discloses an opaque, biaxially oriented film with a cavitated core and transparent skin layers. Colored, light absorbing pigments, such as carbon black or iron oxide, are added to the core and/or the skins in an amount of about 2 to 12 weight per cent to decrease light transmission through the film.
In packaging of all types, there is a need for film which can securely package articles, especially food, and which at the same time is easy to open. Although not restricted to food items such as candy, cakes, fast foods or the like, it is particularly important to the food industry to have facilely openable packaging. Such packaging can be provided by using peelable films. As may be readily appreciated, no matter what is packaged, the consumer does not want to have to struggle with the packaging.
Therefore, there is a need for a film structure of for use in food packaging or the like which is easy to open by virtue of its peelable characteristics.